In voltage power conversion circuits, electrical power can be converted from an alternating current (AC) power into direct current (DC) power via a power converter or AC/DC converter. The DC power can then be converted back into AC power, but into AC power with a desired frequency, phase, and amplitude. The generated AC power is then applied to drive an external load, such as an electric motor.
A three-phase electric drive circuit may comprise a HRG system wherein the neutral node N of the three-phase power input is grounded through a high resistance component. The HRG system includes a heavy-duty resistance device between a neutral node and the ground of the input AC source of the system such that the system can operate under single ground faulted condition by allowing a reduced, non-destructive ground fault current. The HRG system therefore allows the drive system to operate under ground fault conditions, and is a preferred grounding method for many applications.
A HRG fault typically generates small amounts of ground current and may not even be noticeable or measurable in motor drive circuits. Because the fault current is often negligible, it is difficult to reliably differentiate faults from large changes of the load. However, because stresses on the component, motor, or cable in a HRG system become higher under a ground fault condition, it is critical to identify the HRG fault when it occurs. The user may be notified and maintenance may be recommended, including providing an indication of where the ground fault exists.
For a HRG system wherein an adjustable speed drive is applied, the ground fault currents at the drive circuit output are highly dependent on the output frequency of the drive circuit. At a low motor speed, the fault current may be lower than the HRG fault threshold and therefore may not trigger a ground fault indication. If both high and low output frequencies are considered, a false trigger may be generated when the cable lengths of the inverter are long.